Electrophoretic and other electro-optic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
One problem in addressing electrophoretic and similar electro-optic display materials is that typically such materials do not exhibit any significant threshold, i.e., even a small electric field will cause the material to change its display state if the small field is sustained for a considerable period of time. Consequently, it is usually not possible to address such materials using a passive matrix addressing scheme such as is often used with liquid crystal displays. Accordingly, such display materials are addressed using (a) a “direct-drive” addressing scheme, in which each pixel is provided with a separate electrode which can be addressed via its own drive line; (b) an active matrix addressing scheme in which each pixel is provided with an associated non-linear element (such as a transistor); or (c) a moving head addressing scheme, in which a head containing a number of individually controllable discrete electrodes is moved across the display, so that each electrode writes one line of the display.
All three of these addressing schemes have disadvantages. A direct-drive addressing scheme is practicable for a textual display (see, for example, International Application PCT/US99/16652, Publication No. WO 00/05704, which describes such a display using 63 electrodes for each text character) but impracticable for a graphic display, since providing the large number of drive lines required for a graphic display would necessitate excessively complex and expensive circuitry. Active matrix addressing schemes require complex and relatively expensive circuitry, the cost of which typically far exceeds the cost of the electro-optic medium itself. Furthermore, since the cost of producing active matrix addressing circuitry is typically directly proportional to the area of the display, this type of addressing scheme is impracticable for very large displays, for example signs intended as electronic billboards in sports stadia or as message boards in large exhibition halls. Moving head addressing schemes tend to be inexpensive but, since the head needs to be in intimate contact with the electro-optic medium to produce the large electric fields needed to change the display state of most electro-optic materials, the friction between the moving head and the electro-optic material causes wear on the material and limits the life of the display. Furthermore, a moving head is highly sensitive to any slight unevenness in the display material, and in very large signs it is difficult to avoid such unevenness in the display material, especially if the large sign is to be used outdoors, where wind gusts might slightly distort the medium.
The present invention seeks to provide an electro-optic display, and a method for addressing such a display, which are comparable in cost and complexity to a moving head addressing scheme, but which reduce or eliminate frictional wear on the electro-optic material and which are less susceptible to unevenness of the electro-optic material.